As a highly toxic metal ion, Hg2+ causes serious health and environmental problems. Many kinds of chemical and physical sensors have been developed for the detection of Hg2+, among which fluorescence based sensing represents a simple, but sensitive technique providing detection limits of as low as ppb. However, improvement of the detection selectivity in the context of interference from coexisting metal ions remains challenging. Indeed, the concentrations of the common coexisting metal ions are usually much higher than the concentration of Hg2+, for which the safety level set for drinking water by the EPA is only 2 ppb (or 10 nM). To detect such trace amounts of Hg2+ with minimal false positives, a sensor technique with extremely high selectivity can be useful.
Recently, the selective complexation between thymine and Hg2+ has been employed successfully to develop selective sensors for the detection of Hg2+ ions based on fluorescence resonance energy transfer and a colorimetric method. However, both of these sensing systems involve the tedious synthesis of the DNA oligomers and chemical functionalization with different fluorophores (energy donor and acceptor) and nanoparticles. Such processes present a technical hurdle to expedient, cost-effective applications. Moreover, the multiple binding sites within the DNA strands may complicate the chemical process and cause a mismatch in complexation with Hg2+. A DNA strand containing more than four thymine moieties may function as a multidentate ligand that enables effective binding with transition metal ions, such as Zn2+, Cu2+, Ni2+, etc. When these metal ions exist in large excess (as they usually do) compared to the concentration of Hg2+, the binding with Hg2+ will become less competitive, leading to a decreased selectivity for the sensing.